[fix] Removed duplicate import and user iterator to loop over templates

Author: pandafy

openwisp_controller/config/migrations/0056_ow_zt_to_global.py

@@ -1,13 +1,12 @@
 from django.db import migrations
-from django.db import migrations
 
 
 def change_owzt_to_global(apps, schema_editor):
     Template = apps.get_model('config', 'Template')
     updated_templates = set()
     for template in Template.objects.filter(
         type='vpn', vpn__backend='openwisp_controller.vpn_backends.ZeroTier'
-    ):
+    ).iterator():
         if 'zerotier' in template.config:
             for item in template.config.get('zerotier', []):
                 if item.get('name') == 'ow_zt':